Optical element and system using the same

ABSTRACT

An optical element may include a first diffractive structure having a radially symmetric amplitude function and a second diffractive structure having a phase function. The second diffractive structure may serve as a vortex lens. A system employing the optical element may include a light source and/or a detector.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.11/802,044, filed May 18, 2007 now U.S. Pat. No. 7,343,069, which inturn is a continuation of Ser. No. 10/320,525, filed Dec. 17, 2002, nowU.S. Pat. No. 7,221,823 B2, which is a continuation of Ser. No.09/614,184, filed Jul. 11, 2000, now U.S. Pat. No. 6,496,621, which is acontinuation-in-part of U.S. patent application Ser. No. 09/329,996,filed Jun. 11, 1999, now U.S. Pat. No. 6,530,697, which claims priorityunder 35 U.S.C. §119 to Provisional Application No. 60/101,367 filed onSep. 22, 1998, the entire contents of all of which are herebyincorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiment of the present invention are directed to an optical elementhaving both an amplitude diffractive structure and a phase diffractivestructure and/or a vortex lens.

2. Description of Related Art

As the use of non-physical contact connections between light sources andfibers increases, the need for effective isolation to prevent lightreflected at the fiber interface from being returned to the light sourceincreases. Feedback to the light source may result in spectralbroadening, light source instability, and relative intensity noise,which affect the monochromaticity of the light source. As data rates goup, the systems become more sensitive to relative intensity noise andrequire low bit error rates. Conventional optical isolators usingpolarization effects to attenuate reflection are very expensive, makingthe non-physical contact impractical. The importance of avoidingfeedback is further increased when trying to use cheaper light sources,such as vertical cavity surfaces emitting laser diodes and lightemitting diodes.

One solution that avoids the use of an optical isolator is a modescrambler that divides power from the light source into many modes. Aconfiguration employing a mode scrambler includes a single mode pigtailthat provides light from the light source to the mode scrambler thatthen delivers the light to a transmission cable via an air-gapconnector. Since any reflected power will still be divided across themany modes, any reflected power in the mode that can efficiently becoupled into the pigtail is only a small fraction of the total reflectedpower, thereby reducing return losses. However, this solution involvesaligning another fiber, physically contacting the fiber with the modescrambler, and placing the light source against the fiber. Thispigtailing is expensive. Thus, there still exists a need for truenon-physical contact connection between a light source and atransmission system that does not require an isolator.

SUMMARY OF THE PRESENT INVENTION

The present invention is therefore directed to an optical element thatsubstantially overcomes one or more of the problems due to thelimitations and disadvantages of the related art.

In accordance with embodiments, an optical element may include a firstdiffractive structure having a radially symmetric amplitude function,and a second diffractive structure having a phase function.

In accordance with embodiments, a system may include an optical elementhaving a first diffractive structure having a radially symmetricamplitude function and a second diffractive structure having a phasefunction, and a detector receiving light output from the opticalelement.

The first and second diffractive structures may be on a same surface ormay be on opposite surface of a same substrate. A thickness of the samesubstrate may determine a numerical aperture of the optical element. Thesecond diffractive structure may be adapted to output light havingcorkscrew wave. The first diffractive structure may be adapted to focuslight

In accordance with embodiments, a system may include a light sourceoutputting light having a symmetric wavefront, and a vortex lensreceiving light having the symmetric wavefront from the light source andoutputting light having a corkscrew wave.

The vortex lens may be a diffractive optical element. The symmetricwavefront is a planar wavefront. The system may include a mountsubstrate for the light source and an optics substrate for the vortexlens. The system may include a spacer between the mount substrate andthe optics substrate. The spacer and the optics substrate may enclosethe light source. The vortex lens may be a lithograph, e.g., adiffractive optical element.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will bedescribed with reference to the drawings, in which:

FIG. 1 shows the integration of the coupler of the present inventionwith a light source, a fiber and a light source power monitor;

FIGS. 2A-2C illustrate a diffractive element and associatedcharacteristics of a spiral generator for use as the coupler inaccordance with one embodiment of the present invention;

FIG. 3 is a schematic illustration of another embodiment of the couplerof the present invention; and

FIG. 4 is a schematic illustration of another embodiment of the couplerof the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a light source 10, here a VCSEL, a coupler 12 and amulti-mode fiber 14 integrated with a power monitor 16 and a reflectivesurface 18 for directing the light into the fiber 14. In particular, thelight source 10 and the power monitor 16 are provided on a substrate 20.Another substrate 22 has the coupler 12 thereon, preferably on the facefurthest from the light source to allow the beam to expand, and asplitting diffractive element 24 which splits off a portion of the lightfrom the light source 10 to be monitored. The substrates 20, 22 arepreferably mounted with spacer blocks 26, which provide the desiredseparation between the substrates 20, 22. The coupler 12 may also beprovided in a common housing with the fiber 14.

The light split off by the diffractive element 24 is directed to thepower monitor 16 to monitor the operation of the light source 10. Thedirected of the light to the power monitor 16 may be achieved byproviding appropriately positioned reflective portions 28. The number oftimes the light to be monitored traverses the substrate 22 is a designchoice, depending on the initial angle of diffraction and the desiredpositioning of the power monitor 16. This monitoring is set forth incommonly assigned U.S. application Ser. No. 09/386,280, entitled “ADiffractive Vertical Cavity Surface Emitting Laser Power Monitor andSystem” filed Aug. 31, 1999, which is hereby incorporated by referencein its entirety for all purposes. Alternatively, the power monitoringmay be realized using an integrated detector, without the need for thedeflecting element, as set forth in commonly assigned U.S. applicationSer. No. 09/548,018, entitled “Transmission Detection for VerticalCavity Surface Emitting Laser Power Monitor and System” filed Apr. 12,2000, which is hereby incorporated by reference in its entirety for allpurposes

The light that is not split off by the diffractive element 24 proceedsto the coupler 12. A reflective surface 18, such as a polished angularface of another substrate, is provided to direct the light from thecoupler 12 into the multi-mode fiber 14. Preferably all the opticalelements are formed lithographically and all the elements are integratedon a wafer level.

In accordance with the present invention, the coupler 12 is adiffractive element that matches the phase as well as the intensitydistribution of the beam. The matching of the phases generates spiralpropagation of the beam through the fiber. This spiral or vortexpropagation maintains the intensity profile input to the fiber along thefiber. Since the beam travels in a corkscrew, the amount of lightcrossing the center of the fiber is significantly reduced. Ideally, theamount of light in the center will be zero, but in practice, the amountof light is on the order of 10% or less. In contrast, when only theintensity distribution is controlled, as in the first two designs of theparent application, the input intensity profile may be the desiredprofile, but will quickly degrade as the light traverses the fiber, Inother words, while the other designs may provide an input profile thatis substantially null on axis, this profile is only maintained for thedepth of focus of the coupler. When also matching the phase, thisprofile is maintained substantially beyond the depth of focus of a lenshaving the same numerical aperture as the beam to be input to the fiber,e.g., at least an order of magnitude longer. Absent the fiber, the nullspace of the beam profile is maintained through free space, whichsignificantly reduces the alignment requirement. Further, by matchingthe phase and amplitude of the beam to a certain mode of the fiber,theoretically the beam profile could be maintained over an infinitelength of fiber. However, imperfections in the real world, e.g., in thefiber, in the beam, in the matching, degrade from this theoreticalscenario.

Thus, in order to avoid low order modes in a GRIN fiber launch, theamplitude and phase of the higher order modes need to be matched. Thefollowing equations are set forth in Fields and Waves in CommunicationElectronics, Simon Ram et al. 1984, particularly pp. 765-768, which ishereby incorporated by reference in its entirety. For a GRIN fiber,these eigenmodes all have the form set forth in Equation (1):E(r,θ,z)∝ƒ_(mp)(r)e^(±jmθ)e^(±jβ) ^(mp) ^(z)  (1)

where ƒ(r) is a function that depends only on r for given modes within aspecific fiber, r is the radius from the axis, θ is the angle from theaxis, z is the distance along the axis, m is the azimuthal mode number,β is a propagation constant, p is the radial mode number. When m, p=0,the beam has a Gaussian profile.

While Equation (1) could be used to match a particular mode of the fiberby creating an input light beam having an amplitude and phase functionwhich exactly correspond to the particular mode, such matching is notrequired and may not even be desirable, as matching the amplitude aswell as the phase increases the requirements on the optics. As long asm>0, the azimuthal mode m will have a phase function that is a spiralring, whether the light is traveling in free space or in a fiber. Oncethe phase function for at least one higher order mode, i.e., m>0, hasbeen matched, a null at the center of the beam is created after the beamhaving been phase matched propagates over a short distance, e.g., a fewwavelengths. Unlike other types of matching, this null is maintained inthe center in both free space and the fiber, so such an optical elementproviding such matching does not have to be immediately adjacent to thefiber. As evident from Equation (1), when matching the phase, the valueof p doesn't matter.

In order to suppress the lowest order mode, i.e., m=0, a phase termneeds to be added to the wavefront. This is accomplished through the useof the following diffractive phase function encoded onto the wavefrontset forth in Equation (2):

$\begin{matrix}{{\phi( {x,y} )} = {m\;{\arctan( \frac{y}{x} )}}} & (2)\end{matrix}$

where φ is the phase function, x and y are the coordinates in the plane.In general, there will be several modes propagating, e.g., m=1-5. Thespiral mode may be realized by matching the phase function for m=3.

This phase function can be added to a lens function and encoded as amod(2π) diffractive element as set forth in Equation (3):

$\begin{matrix}{{\phi( {x,y} )} = {\frac{\pi( {x^{2} + y^{2}} )}{\lambda\; f} + {m\;\arctan\;( \frac{y}{x} )}}} & (3)\end{matrix}$

FIG. 2A illustrates the mod(2π) diffractive element and thecorresponding intensity to in the focal plane of the lens function. FIG.2B illustrates an actual example of a diffractive optical element 12created in accordance with Equation (3). FIG. 2C illustrates thesimulated ring intensity 25 and the measured intensity pattern 29 of theelement 12 in FIG. 2B. A refractive equivalent in accordance withEquation (3) of the phase matching diffractive 12 may be alternatelyemployed.

This phase matching coupler 12 is not a true beam shaper, since eachpoint in the input plane is mapped into more than one point in theoutput plane because of the axial singularity. Unlike a diffuser, eachpoint in the input plane is not mapped to every point in the outputplane.

The phase matching coupler 12 allows the desired angular distribution tobe substantially maintained along a portion of the fiber. This may bequantified by measuring the amount of power within a certain radius ofthe fiber at a certain distance along the fiber. The phase matching ofthe present invention allows more power to be contained within thedesired radii for a longer distance than methods not employing phasematching. For example, by aligning the coupler and a GRIN fiber alongthe same axis, using a 850 nm source, and matching both the phase andthe amplitude, the encircled energy can be maintained to less than 12.5%is a radius of less than 4.5 microns and 75% for a radius less than 15microns, with no power in the fiber center, for over 6 m.

By matching the phases, the light from the coupler is input to the fibertraveling in a circular direction, i.e., the path of the light down thefiber forms a corkscrew. Such traversal is opposed to the linear travelnormally occurring down the fiber. By traveling in a corkscrew or spiralmode, the input distribution, typically annular, of the input light ismaintained along the fiber. Without the phase matching, while theinitial input light has the desired shape, this shape is not retainedthroughout the traversal of the fiber. Therefore, more modal dispersionwill be present, with more light in the center of the fiber, if phasematching is not used.

In addition to efficiently coupling the light into the fiber, the phasematching coupler 12 also reduces the power being fedback into the lightsource 10. Since the phases are matched, and the reflected light willnot have the same phase as it did when originally incident on the phasematching coupler 12, the phase matching coupler 12 will not return thelight back to the light source as it came. In other words, when thereflected light traverses the system, it will be further deflected bythe phase matching coupler 12, thereby reducing the power fedback intothe light source 10.

The back reflection reduction of the phase matching coupler onlyoperates sufficiently when the phase matching coupler 12 is far enoughaway from the fiber so that the phase is sufficiently changed to preventbeing redirected in the same manner. In other words, if the phasematching coupler 12 is placed in contact with the end of the fiber,while the coupler will still serve to maintain the input distribution,since the reflected light will have essentially the same phase as theinput light, the light will be returned substantially back to the lightsource as it came. However, if the phase matching coupler 12 is placedat least roughly three times the diameter of the beam incident on thefiber, there is sufficient alteration of the phase due to traversal thatthe reflect light incident on the phase matching coupler 12 will befurther deflected.

Further reductions to the amount of light being fedback to the lightsource 10 may be realized by using a lens 30 in addition to the phasematching coupler 12 as shown in FIG. 3. This lens 30 is used to shapethe light to provide additional reduction in the power fedback to thelight source. The lens 30 is preferably a diffractive surface that is acombination of a lens function having radially symmetric terms with anegative axicon function. When the phase matching coupler 12 is spacedaway from the fiber, the lens 30 may simply form a ring, since the phasematching coupler will prevent the light from being retraced. As shown inFIG. 3, the lens 30 is on a first surface 34 of a wafer 32. The phasematching coupler 12 is provided on a second surface 36 of the wafer 32,opposite the first surface. The thickness of the wafer 32 controls thenumerical aperture of the image. Alternatively, the phase matchingcoupler 12 may be formed on the same surface as the lens 30.

The lens 30 allows an annular intensity ring to be optimized for theparticular fiber 14. Also, by forming this ring prior to the phasematching coupler 12, a smaller radial segment of the phase matchingcoupler is used. As can be seen from equation (2), as m increases, theamount of phase twist increases. Thus, rays at the center of the phasematching coupler 12 receive a larger skew angle that rays at the edge ofthe phase matching coupler. By shaping the light into an annulus, thiscentral portion is avoided, reducing the aberrations introduced by thephase matching coupler 12. Again, the light reflected back from thefiber 14 will not have the same phase as the light incident on the phasematching coupler 12, so the light will be further deflected by the phasematching coupler 12. Since the deflection angles are now altered fromthat of the light source, the lens 30 will not focus the light back ontothe light source, but will further deflect the light away from the lightsource.

Another embodiment is shown in FIG. 4. Here, the phase matching coupler12 is not used, only a reciprocal, phase sensitive system 40. An opticalelement will map an optical distribution, i.e., amplitude and phasedistribution in an input plane to an output plane. If an optical elementis a reciprocal optical, it will map the same optical distribution in anoutput plane back to the original optical distribution in the inputplane, as long as the light has the same phase and intensity profile.Optical systems that perform one-to-one mapping, such as an imaginglens, are reciprocal, but are also phase insensitive when performing amapping between an object plane and an image plane, i.e., a change inphase will not affect the mapping between the image and object planes.However, other optical systems, such as those that perform a one to manymapping, i.e., in which one point in the input plane is mapped to morethan one point in the output plane, while reciprocal, are typicallyphase sensitive. In other words, a phase change will alter how the lightin the output plane is returned to the input plane. An example of such asystem is a negative axicon.

In the preferred embodiment, this system 40 also creates an intensityring on the plane at which the fiber 14 is located. The reflection fromthe fiber creates a ring back onto the system 40, but the phase of thelight has been altered due to the reflection. This change in phaseresults in the light traversing the system 40 having an increaseddiameter of the ring in the object plane, rather than returning the ringto the point source of the light source. This increased diameter resultsin most of the light missing the input of the light source,significantly reducing feedback. Any other reciprocal, phase sensitivesystem that results in most of the light avoiding the light source maybe used. The phase matching coupler 12 may still be employed in anyposition to increase coupling bandwidth and/or enhance the feedbacksuppression.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

1. A device, comprising: a light source; and an optical systemconfigured to transfer light from the light source to an optical fiber,the optical system including a diffractive surface defined by a surfacefunction having a radially symmetric phase function and an angularlysymmetric phase function, the surface function being selectivelyadjustable to control launch conditions and manage reflections.
 2. Thedevice as claimed in claim 1, wherein the radially symmetric phasefunction is a lens function.
 3. The device as claimed in claim 2,wherein the lens function is given by$\frac{\pi( {x^{2} + y^{2}} )}{\lambda\; f},$ where ƒ is thefocal length of the lens function and λ is a wavelength of the lightsource.
 4. The device as claimed in claim 1, wherein the angularlysymmetric phase function is given by${m\;{\arctan( \frac{y}{x} )}},$ where m is a mode numberindicating a speed with which a phase of the angularly symmetric phasefunction is changing.
 5. The device as claimed in claim 4, wherein mequals
 3. 6. The device as claimed in claim 1, wherein the radiallysymmetric phase function is a polynomial.
 7. The device as claimed inclaim 6, the polynomial is a polynomial expansion.